The invention relates to nuclear fuel rods for use in nuclear reactors and comprising a gas tight sheath containing a stack of fuel pellets and means for holding the stack against an end plug of the sheath. The term "fuel" should be interpreted widely as designating not only materials used for producing energy by fission, but also fertile or neutron absorbing material.
The fuel rods used up to present, particularly in reactors cooled by circulation of water or molten sodium, comprise a stack of oxide fuel pellets and holding means formed by a helical spring between the stack and an end plug. This solution has considerable drawbacks. The spring requires space and reduces the volume available for receiving the fission gases released by the fuel. Under irradiation, the material forming the spring is subject to relaxation which causes them to lose a fraction of their initial properties. Often, degradation during the operation in a reactor is such that the holding means no longer provide sufficient retention during fuel handling and transportation.
Attempts have been made to solve this problem. French Pat. No. 2,529,371 describes a fuel rod whose holding spring is made from a material which exerts a retaining force which decreases when the temperature increases. The only advantage of this solution is that it reduces the stresses undergone by the fuel pellets during operation in the reactor. French Pat. No. 2,018,665 describes a rod in which the spring in replaced by a member in the form of a split bucket resiliently engaging the sheath. Such a member has drawbacks: it exerts a constant retention force and remains in position when the stack of pellets contracts, so that it no longer holds the stack in position during shipping.